In all pictures of the P-51 on assembling lines it can be noticed that the flaps and ailerons are of the shiny metal skin and were left unpainted. Contrary to that the rest of the wing is either of the aluminium matt paint or a kind of the grey that is also matt and then painted with the silver. So the unpainted parts of the NMF should look slightly like a mirror. I mean a little bit shiny and cristal silver. In time these were getting more of the mattish/grey tone due to the oxidation.